Trunk lines in modern telecommunications networks are typically arranged in a bandwidth hierarchy in which each type of trunk line is capable of carrying data from multiple hierarchically lower types of trunk lines. For example, in North America, Type 1 (T1) trunk lines are operated at a bitrate of 1.544 Mb/s (megabit per second) and can communicate up to twenty-four 0.064 Mb/s voice frequency (VF) signals at a time. The twenty-four VF signals are sampled one after another, digitized to respective bytes and time-division multiplexed onto the T1 carrier to generate each frame of a Type 1 Digital Signal (DS-1). Type 3 (T3) trunk lines are operated at a bitrate of 44.736 Mb/s (megabit per second) and are capable of time-division multiplexing up to twenty-eight DS-1 signals in a single Type 3 Digital Signal (DS-3). Even higher bandwidth trunk lines in the TDM hierarchy are available to multiplex DS-3 signals from multiple T3 lines.
To reduce the number of physical connections between trunk lines and signal forwarding equipment (e.g., trunk multiplexers, network routers and network switches), it is common to connect a relatively small number of high bandwidth trunk lines to the signal forwarding equipment instead of a larger number of lower bandwidth trunk lines. For example, instead of connecting 84 T1 cables to a signal forwarding device, three T3 cables may be connected instead. The 84 DS-1 signals that otherwise would have been received on discrete T1 lines are extracted from the DS-3 signal on the incoming T3 line.
One problem that arises when relatively high bandwidth trunk lines are connected directly to a signal forwarding device is that of distributing the extracted signals throughout the device. Signal forwarding devices typically include multiple service modules that are used to handle incoming signals in parallel. In many cases the service modules are removably connected to a backplane (or midplane) of the signal forwarding device and signals are passed between the service modules via traces in the backplane. When incoming signals are concentrated at one service module and then distributed to other service modules, the required number of backplane traces is increased significantly. For example, if 84 DS-1 signals extracted from three T3 lines on one service module are to be serviced by other service modules in a signal forwarding device, 84 additional backplane traces are required. To make matters worse, when trunk signals are extracted from higher frequency trunk signals, it is common to generate a separate clock and data signal for each extracted trunk signal. Thus, in the example above, up to 168 additional backplane traces may be required to distribute the clock and data for the 84 DS-1 signals. In many devices, physical constraints on the density of backplane traces prevent this amount of signal distribution.
One possible solution to the problem of wire trace congestion on the device backplane is to increase the data transmission frequency to obtain more bandwidth out of each backplane trace. However, due to the frequency response characteristics of signal traces on the backplanes of many devices, it is not possible to substantially increase signal transmission frequency without unacceptable loss of signal quality.